Singing is when you make musical sounds with your voice. When you make a sound and you hold that note. One of the best ways to describe it is that it is sustained speech. So if you can speak, you can sing.
You can either sing with musical instruments, or acapella, which is just singing without accompaniment. There are many different benefits of singing, which you can find here.
Musicians can develop their voices through systematic practice of both songs and vocal exercises.
Basically, anyone can sing as long as you’re willing to put in work into it.